Gyratory compactors are used to test material properties, such as hot-mix asphalt (HMA) mixture specimens in the laboratory to assess and predict paving material performance. For HMA testing, Federal Highway Administration (FHWA) guidelines and test procedures require in essence that an asphalt material sample be gyrated within a mold while undergoing compaction, to simulate the effect of road traffic. The extent of compaction according to the testing procedures is controlled by the force applied to a compaction ram, the angle of gyration or orbiting of a cylindrical mold which holds a material specimen “gyration angle”, and the number of gyrations. The angular extent of gyration is determined by an angle of the mold relative to the axis of the compaction ram. Precise dynamic measurement of the gyration angle is critical to achieve accurate test results, and is complicated by the constant gyration/orbiting motion of the mold as material is compacted within it.
Gyratory compactor machines typically include a cylindrical material holding mold which is gyrated by a gyration mechanism and positioned for insertion of a ram into the mold cavity to compress the material in the mold as the mold is gyrated. Representative gyratory compactor machines include those manufactured by Pine Instrument Company and described in U.S. Pat. Nos. 5,456,118; 5,606,133; 5,824,913 and 5,817,946, and other types of such machines which perform these functions. The material-holding mold may be gyrated about a small angle relative to the vertical axis of the ram (the gyration angle), by orbiting one end of an axis of the mold as the material is compressed within the mold by the ram to simulate actual forces on the material in the application environment. Specimens of a consistent density are produced when a gyratory compactor maintains a constant pressure and a known constant angle of gyration during the compaction process. Inconsistencies in asphalt specimens produced on different gyratory compactor models have been attributed to variations in the angle of gyration.
Some gyratory compactors measure the angle of gyration by referencing the external cylindrical mold wall to a reference frame, such as a vertically oriented member of the compactor machine frame. However, the asphalt mixture being compacted is contained on the inside of the mold, being compressed by mold end plates, pucks and/or ram heads. It is the angle between the mold cylinder and these internal compaction surfaces of the end plates that determines the amount of compaction effort imparted onto the HMA specimen. This angle, referred to herein alternatively as the “internal angle”, “internal angle of gyration”, or “internal mold gyration angle”, must be accurately maintained throughout a gyratory compaction test procedure in order to achieve the proper degree of compaction and accurate test results. It is therefore important to have information regarding the internal gyration angle for each test. A typical gyratory compactor utilizes a mold comprising of a mold body of substantially cylindrical shape, referred to herein as the mold cylinder, and at least one end plate, but preferably two end plates. In such a compactor, there are actually two internal angles of gyration, one at each end of the asphalt specimen. The internal angle of gyration is measured at each end of the asphalt specimen and the average of the two measurements is used to establish the effective internal angle. Although the internal angles at each end of the specimen are of interest, it is the effective internal angle of gyration that is of primary interest.
Measurement and display of the internal angle of gyration, vital to obtaining accurate tests results, is a feature not adequately addressed in the design of prior art gyratory compactors. In prior art gyratory compactors, when operated without an internal angle measurement device, the gyration angle is set prior to running a test on a material specimen. It is then simply assumed that the machine maintains this angle throughout the test operation. It is further assumed that the test results are based upon the pre-set gyration angle being maintained throughout the gyratory compaction process. Machines which do not maintain the internal angle of gyration in operation produce inaccurate test results.
There has been developed a device which measures the internal angle of gyration, by placing within the mold at least two probes which contact the inner mold wall. The data retrieved from the probes, along with the known spacing between the probes, is used to calculate the internal angle of gyration with respect to the mold base plate or top plate. The instrument interfaces with a computer through cable connection to setup the data collection parameters and to retrieve the data once the compaction test is complete. The instrument does not provide or display internal gyration angle data during the compaction test, which would allow a machine operator to immediately tell whether a particular test was being performed within the specified parameters. Furthermore, the use of this separate angle validation device requires separate insertion of the device into a gyratory compactor in order to conduct a test from which angle data can be acquired.